Abstract
Abstract. The EISCAT VHF radar (69.4°N, 19.1°E) has been used to record vertical winds at mesopause heights on a total of 31 days between June 1990 and January 1993. The data reveal a motion field dominated by quasi-monochromatic gravity waves with representative apparent periods of ~30–40 min, amplitudes of up to ~2.5 m s–1 and large vertical wavelength. In some instances waves appear to be ducted. Vertical profiles of the vertical-velocity variance display a variety of forms, with little indication of systematic wave growth with height. Daily mean variance profiles evaluated for consecutive days of recording show that the general shape of the variance profiles persists over several days. The mean variance evaluated over a 10 km height range has values from 1.2 m2s–2 to 6.5 m2s–2 and suggests a semi-annual seasonal cycle with equinoctial minima and solsticial maxima. The mean vertical wavenumber spectrum evaluated at heights up to 86 km has a slope (spectral index) of –1.36 ± 0.2, consistent with observations at lower heights but disagreeing with the predictions of a number of saturation theories advanced to explain gravity-wave spectra. The spectral slopes evaluated for individual days have a range of values, and steeper slopes are observed in summer than in winter. The spectra also appear to be generally steeper on days with lower mean vertical-velocity variance.Key words. Meteorology and atmospheric dynamics (middle atmosphere dynamics; waves and tides)
Highlights
Gravity waves play an important role in the dynamics of the middle atmosphere
We have presented analysis of 31 days of vertical-wind records made using the EISCAT VHF radar between 12 June, 1990 and 25 January, 1993
The motion ®eld appears to be dominated by high-frequency motions of large vertical wavelength and periods of order 30±40 min
Summary
Gravity waves play an important role in the dynamics of the middle atmosphere. The waves are known to Correspondence to: N. A number of theories invoking dierent physical mechanisms have been advanced to account for the observed spectra, including theories based on linear instability (Dewan and Good, 1986; Smith et al, 1987), diusive damping (Weinstock, 1990), Doppler spreading (Hines, 1991), saturated-cascade processes (Dewan, 1994) and diusive ®ltering (Gardner, 1994) These theories are all successful in accounting for the observed spectral amplitudes and slopes when the spectra are those ofuctuations in horizontal velocity or temperature, and so observations of these quantities cannot be used to distinguish between the theories. In the present study a much larger data set of lower time-resolution measurements made on a total of 31 days is used to investigate a number of properties of the vertical motions due to gravity waves at mesopause heights. Spectra of vertical velocities in terms of vertical wavenumber are presented and the availability of data from dierent seasons, and from short runs of consecutive days of recording, allows an investigation of the seasonal and day to day behaviour of some aspects the vertical-velocity ®eld
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